Research Stories

by Jodi Guyot

Your mother always told you not to use your teeth as tools to open something hard, and she was right. Human skulls have small faces and teeth and are not well-equipped to bite down forcefully on hard objects. Not so of our earliest ancestors, say scientists. New research published in the February 2009 issue of Proceedings of the National Academy of Sciences reveals nut-cracking abilities in our 2.5-million-year-old relatives that enabled them to alter their diet to adapt to changes in food sources in their environment.

Mark Spencer, an Arizona State University assistant professor, and doctoral student Caitlin Schrein in ASU's School of Human Evolution and Social Change, are part of the international team of researchers who devised the study featured in the article "The feeding biomechanics and dietary ecology of Australopithecus africanus." Using state-of-the-art computer modeling and simulation technology—the same kind engineers use to simulate how a car reacts to forces in a front-end collision—evolutionary scientists built a virtual model of the A. africanus skull and were able to see just how the jaw operated and what forces it could produce.

"We started with a CT scan of a skull that is one of the most complete specimens of A. africanus that we have," said Spencer, researcher in ASU's Institute of Human Origins and a lead investigator on the project, which was funded by the National Science Foundation and European Union. This would be a later ancestor of Lucy—STS5—who is affectionately known as "Mrs. Ples." The skull, discovered in 1947, has struts on the side of the nose, but no teeth.

"We meshed those data with another specimen with teeth to make the virtual model of the bone and tooth structure. Then we looked at chimpanzees, who share common features with Australopithecus, and took measurements of how their muscles work and added that to the model. We were able to validate this model by comparing it to a similar model built for a species of monkey called macaques," Spencer explained.

The result—a rainbow-colored virtual skull that illustrates forces absorbed by the cranial structure in simulated bite scenarios and how their unusual facial features were ideally suited to support the heavy loads of cracking hard nuts.

"It was like watching ‘Mrs. Ples' come to life," Spencer said.

"This reinforces the body of research indicating that facial specializations in species of early humans are adaptations due to a specialized diet," said Spencer. "The enlargement of the premolars, the heavy tooth enamel and the evidence now that they were loading forcefully on the molars suggest the size of the objects were larger than the previously hypothesized small seeds and nuts.

"These fall-back foods—hard nuts and seeds—were important survival strategies during a period of changing climates and food scarcity," he added. "Our research shows that early, pre-stone tool human ancestors solved problems with their jaws that modern humans would have solved with tools."

For more information contact Mark Spencer, School of Human Evolution and Social Change, College of Liberal Arts and Sciences, 480.965.6215. Send email to Mark.Spencer@asu.edu